Organ specificity and lactate-dehydrogenase activity. 2. Some properties of human-heart and liver preparations

Abstract

The lactate-dehydrogenase activities of human-heart and -liver preparations have been determined by measuring the rates of reduction of pyruvate and of 2-oxobutyrate with NADH2 as coenzyme, and also by means of the reverse reaction. For both reactions the 2-oxobutyrate activity/pyruvate activity ratio for heart is about three times as great as that for liver. Temperature coefficients, thermal stabilities and apparent enzyme-substrate constants (Ks) have been determined for various human-tissue preparations with pyruvate and 2-oxobutyrate. Oxamate competitively inhibits the reduction of 2-oxobutyrate by both heart and liver preparations to a much greater extent than that of pyruvate. With a given tissue preparation, oxalate inhibits the reduction of both substrates equally. A relationship has been established between the reciprocal inhibitor constant (1/Ki) and the activity ratios for human heart, serum, skeletal muscle and liver.